The Schmitt trigger shown in Figure 2 works by
comparing the input voltage to a feedback loop from the
output so that once it goes high, it stays there until the input
falls below the threshold. There are a number of good
references for determining the resistor values and

http://hyperphysics.phyastr.gsu.edu/hbase/electronic/
schmitt.html#c2 even provides an interactive web page to
calculate the resistor values.

There are two equations that determine the V2: the
voltage it triggers; and V'2 – the voltage at which it will
un-trigger. By having the V'2 and not just a trigger voltage,
you reduce rapid cycling at the trigger voltage. First, you
want to pick your V2 and V'2 and then use the equations to
calculate the required value of each resistor. Also, you can go
to the website mentioned above and use the resistor and
voltage calculator they provide. The catch is it only works if
you already know the resistors you want to use and then finds
the V2 and V'2 values.

R1, R2, and R3 represent the three resistors
used for the Schmitt trigger. In the equation
above, R123 = R1||R2||R3 which really means R1
R2 R3/ ( ( R1*R2) + (R1 R3) + (R2 R3)).
We wanted a V2 trigger voltage around 3V
and a V2' of2V. So we plugged that into the
equations:

We found that we needed 2.5K, 2.0K, and
10K resistors. Given that, the closest one we
had was a 2.2K. We used that for R1 and R2,
which gave us a V2 and V'2 of 2.75V and
1.76V.

Enough with this boring math, let’s build
it already!

Parts You Will Need

With the exception of the Vex parts, all the components
required for the Schmitt trigger were easily found at our local
RadioShack.

The Schmitt trigger does not have many
pieces to it and is very easy to assemble after you
have worked the equations. Figure 3 shows how
we put the Schmitt trigger on a circuit board,
leaving room for other circuits. You can just as
easily plug the parts on to a protoboard.

Plugging the encoder into the board presented
an obstacle. Since we didn't want to damage the
shaft encoder, we used an extension cable which
we customized by cutting off the male plug end,

50 SERVO 10.2008

stripping the ends of the wires, and soldering them to the
protoboard. This left the shaft encoder completely unharmed
and detachable. Also, the newer shaft encoders in the Vex
Robotics kit have two outputs, so it doesn't matter which one
you connect to the board.